Technical Field
The present invention relates to needle coke useful for applications including producing graphite electrodes. More particularly, the present invention relates to a process for producing needle coke from a coal tar distillate as a starting material for graphite electrodes which exhibit a reduced coefficient of thermal expansion.
Background Art
Carbon electrodes, especially graphite electrodes, are used in the steel industry to melt both metals and supplemental ingredients used to form steel in electro-thermal furnaces. The heat needed to melt the substrate metal is generated by passing a current through at least one and, more commonly, a plurality of electrodes and forming an arc between the electrodes and the metal. Currents in excess of 100,000 amperes are often used.
Electrodes are typically manufactured using a needle coke filler combined with a pitch binder. Needle coke is a grade of coke having an acicular, anisotropic microstructure. For creating graphite electrodes that can withstand the ultra-high power throughput, the needle coke must have a low electrical resistivity and a low coefficient of thermal expansion (CTE) while also being able to produce a relatively high-strength article upon graphitization. The CTE value assigned to a needle coke is conventionally determined by admixing the milled, calcined coke with a pitch binder, extruding the coke/pitch blend to form an electrode, followed by heat treatment of the electrode to about 3000° C. to graphitize the electrode. The CTE value is then measured on the graphitized electrode.
The specific properties of the needle coke are determined by the choice of feedstock and the control of parameters in the coking process in which an appropriate carbon feedstock is converted into needle coke. Typically, the classification of needle coke is through a system of grade levels, which are distinguished as a function of the CTE over a certain temperature range. For example, premium needle coke is usually classified as having an average CTE of less than about 1.00×10−6/° C. over the temperature range of from about 100° C. to about 400° C. while regular grade needle coke has an average CTE of from about 1.00×10−6/° C. to about 1.25×10−6/° C. over the temperature range of from about 100° C. to about 400° C. The CTE value of the graphitized electrode produced with the coke filler is measured in the extruded (i.e., longitudinal) direction using either a dilatomer or the capacitance method as described in G. Wagoner et al., Carbon Conference 1986 Proceedings, pp. 234, Baden-Baden, 1986.
To convert the needle coke to graphite, the article containing the needle coke (e.g., the electrode) should be heated generally in a range of from about 2000° C. to about 3500° C. to convert the needle coke to a graphitic crystalline structure while eliminating volatilizing impurities. Such impurities negatively increase the CTE of a formed graphite electrode, and can result in electrode expansion as current is applied. The expansion will alter the arcing properties of the electrode either rendering the process less efficient or possibly resulting in electrode breakage.
Low CTE needle coke suitable for high performance graphite electrodes is largely produced from petroleum-derived feedstocks. For this purpose, the feedstock should be highly aromatic, provide a good carbon yield after coking, and be very low in ash and infusible solids. Typically in a production of petroleum needle coke, fluid catalytic cracking (FCC) decant oil is used as a starting material which contains about 0.02% to about 0.04% by weight of ash. The major constituent of ash is FCC catalyst remaining from the original cracking of the decant oil. This FCC catalyst increases the thermal expansion characteristics of a resulting electrode, thereby necessitating the removal of the catalyst for production of low CTE graphite electrodes from petroleum needle coke. As a result, many individuals have developed methods for removing the ash particles so as to decrease the CTE of the resulting electrode. For example, in U.S. Pat. No. 5,695,631, Eguchi et al. discloses a method for producing petroleum needle coke which includes filtration, centrifugation, and/or electrostatic aggregation to remove a substantial portion of the FCC catalyst from the decant oil.
While the use of petroleum-based needle coke can result in the formation of a graphite electrode with a lower CTE, there are significant disadvantages to using petroleum-based needle coke. One such disadvantage is the potential shortage of petroleum-derived needle coke as the price of petroleum continues to rise. Furthermore, there are few and limited suppliers of petroleum needle coke suitable for the creation of low CTE graphite electrodes. Additionally, the cost of petroleum needle coke is pushed even higher due to the required filtration to remove a significant portion of ash from the decant oil.
A different approach is to use coal-based feedstocks in providing needle coke for graphite electrodes. In this process, coal tar is derived from the coking process used to produce metallurgical coke from coal. The coal tar is obtained as the overhead product and contains infusible carbonaceous solids formed by gas-based carbonization and also as a result of coal carryover. These remaining solids interfere with the development of a large domain mesophase when forming needle coke and instead result in the formation of a high CTE coke.
Despite these solids, coal tar would be a desirable starting material for producing coke because coal tar is highly aromatic and has a high carbon yield. Coal tar generally has carbon yields of from about 10% to about 30% as determined by a modified Conradson carbon (MCC) test. However, in order to obtain a low CTE coke from coal tar, a physical solid separations process must be employed to remove undesirable solids which constitute up to 10% of the tar.
Examples in which solids have been removed from coal tar for the preparation of needle coke include Japanese Patent No. JP19850263700, Misao et al, in which quinoline-insoluble components are removed from coal tar and/or coal tar pitch for the use in delayed coking to produce needle coke.
In Masayoshi et al. (German Patent No. DE3347352), a process is described for producing needle coke in which hydrogenation is used to remove solid components from the coal tar. Specifically, coal tar and/or coal tar pitch raw material, is purified by hydrogenation in the presence of a hydrogenation catalyst until a denitrification ratio of at least 15% by weight is reached.
Unfortunately coal tar-based needle coke produced by the prior art includes steps which are costly while also creating the issue of disposing the mixture of residual tar and the separated solids. The use of a variety of different catalysts for the removable of solid components of coal tar can create waste products which cause both economic concerns and also environmental issues which factor into the cost of utilizing coal tar for the production of low CTE needle coke. Furthermore, many prior art processes require significant energy input as high temperatures are often necessary for the catalyst to remove a substantial amount of solids from the feedstock. Furthermore, when the hydrogenation catalyst is utilized in removing solid components from the coal tar, a constant stream of hydrogen is necessary for the reactive process to function. Such a hydrogenation process also lowers the overall coke yield of the feedstock by reducing aromaticity.
Rather than utilizing coal tar, processes have developed which utilize coal tar distillates to produce mesophase pitch. Lewis et al., U.S. Pat. No. 4,317,809, describe a process in which a coal tar distillate is heated at 750 psig for 5 hours at 450° C. to form a mesophase pitch. The overall yield of mesophase pitch is lower than desired, and the pressure utilized is considered too high for use in a commercial delayed coking process which generally operates below about 100 psig.
What is desired, therefore, is a process for producing needle coke for low CTE graphite electrodes which does not require the use of a petroleum-derived feedstock, and therefore, would not contain ash which increases the CTE of the resulting graphite electrode. Furthermore, a process is desired which eliminates the infusible carbonaceous solids which are present in coal tar and require removal for the production of a low CTE graphite electrode from coal tar. Indeed it would be desirable to have a process for directly converting the coal-based precursor to highly anisotropic needle coke without the need for a solid separation process which could also be readily adaptable to commercial delayed coking processes. It would also be desirable to have a process which results in a high coke yield while using lower pressures than are currently standard to commercial refinery coking operations.